Marine oil and gas resources have become an important part of the global energy strategy at present, and the deepwater areas will become the main territory for oil and gas resource exploration and development in the future. However, the deepwater areas have extremely adverse environmental condition, which places higher demands on deepwater drilling equipment. In the engineering of mining the marine oil and gas resources, a marine riser is a key device that connects a floor and a subsea wellhead, which needs to bear the coupling effects of the marine environments and drilling conditions, and is prone to such accidents as wear, fatigue fracture and the like. Major economic losses and environmental security problems due to marine riser accidents have been caused for multiple times at home and abroad. The marine riser isolates an oil well from the outside seawater, supports various control pipelines, provides a channel for circulation of drilling fluids, and offers guidance for the drilling work of a drilling rod from the drill floor to the subsea wellhead. Therefore, failure of the marine riser will cause damage to drilling vessel, subsea equipment and oil well to result in great economic losses. In addition, the leakage of the drilling fluids and oil will also cause severe environment contamination.
Meanwhile, with the development of marine drilling towards deepwater and ultra-deepwater and ever-increasing slenderness ratio of the marine riser, the flexible features become more apparent, and the top tensions actually applied at the two ends of the marine riser in the engineering are increased therewith. In addition, the dynamic response of the marine riser to the self vibration thereof causes periodic changes to the axial forces born by the upper and lower boundaries of the marine riser. Therefore, the axial force bearing feature of the marine riser places higher demands on the axial intensity of the marine riser. The huge span of the marine riser on a direction vertical to the sea level makes the transverse modification of the marine riser be increased greatly under the joint action of wind waves and currents. Moreover, such vortex-induced vibration of the marine riser as ocean current, wave, wind load and the like are more important reason of the fatigue failure thereof. The seawater while flowing through the marine water will form alternate shedding vortex at the two sides of the marine riser body, thus inducing the periodic vibration of the marine riser, while the vibration of the marine riser will further disturb shedding of current field vortex. When the shedding frequency of the vortex is approximate to the natural frequency of the marine riser, a locking phenomenon will occur, and the structure of the marine riser resonates largely thus accelerating the fatigue failure of the marine riser.
Presently, studies on marine riser are approximately divided into three broad categories: test method, numerical method and semi-empirical formula. For the test method, the axial force bearing changes, the lateral load and force bearing changes as well as lateral displacement and real time strain changes of the marine riser are complicated and volatile change process. Moreover, the vortex-induced vibration caused by vortex shedding is a multi-physics coupling interacted complicated process. The more prominent for the petroleum engineering deepwater drilling is that: excluding such marine working conditions as wind, wave, current and the like, those drilling conditions as circulation of the drilling fluids in the annular part of the interior of the marine riser and the collision and friction between the rotation of a drill stem and the marine riser also have a great impact on the mechanical behavior of the marine riser. Therefore, a set of complete physical test scheme and precise test instruments that can synchronously observe all related machine models is needed to truly test and study the mechanical property of the marine riser during the actual production process, so as to determine the joint effect thereof. It is usually very difficult for a physical test to provide the instantaneous change data of the fluids at the same time. Therefore, to comprehensively and truly simulate the working condition of the marine riser is the premise for the credibility of the test, and to monitor the instantaneous change of the marine riser and the surrounding current field is the key for the success of the test.
Presently, most studies on the failure of the marine riser at home and abroad focus on the vortex-induced vibration of the marine riser, but neglect that the deepwater drilling process is, an engineering having a shorter period. The fatigue failure doe cause damage to the service life of the marine riser; however, compared with the failure caused by mutations of such load as wind waves and currents, the fatigue failure caused by the vortex-induced vibration of the marine riser has already played second fiddle. Even for the vortex-induced vibration, the studies on the failure of marine riser at home and abroad have carried out vortex-induced vibration tests on the marine risers having different marine working conditions, different slenderness ratios and different materials. The tests on the vortex-induced vibration of the marine riser or riser conducted by most scholars at home and abroad focus the test emphasis on the changes of incoming current types and slenderness ratios as well as span of Reynolds number. For example: Chaplin developed a test on the vortex-induced vibration of a flexible riser under a step current in 2005. Trim et al conducted a test in a Marintek marine towing basin in 2006, obtaining high-quality data under different water current conditions and high mobility response conditions. Zhang Jianqiao from Dalian University of Technology conducted a test on the vortex-induced vibration of a flexible riser at the nonlinear wave tank of the State Key Laboratory of Coastal and Offshore Engineering of Dalian University of Technology in 2009, and the like. However, studies related to the complicated working conditions for the vortex-induced vibration of the marine riser having a big slenderness ratio during the marine drilling process are still insufficient. In 2008, Guo Haiyan on the basis of the original test, optimized the test design, taking the influences of such factors as different tension forces, internal current rates, mass ratios and the like, on the vortex-induced vibration response of the riser into consideration. In 2011, Guo Haiyan further conducted a vortex-induced vibration response test on the riser under the effects of different internal currents, external currents and top tensions in the “Wind-Wave-Flow” Joint Tank of Ocean University of China. The several tests taking the internal current of the riser into consideration cannot simulate the actual working condition of the marine riser in true marine drilling process more comprehensively yet although the simulation about the drilling condition of the marine riser is further improved. Therefore, the studies on the mechanical behavior of the marine riser marine riser are not comprehensive yet.